The present invention relates to a device for feeding an elastically extendable yarn to hosiery knitting machines.
Devices for feeding an elastically extendable yarn to hosiery knitting machines are known.
One of these devices is composed of a yarn accumulation device, which is substantially constituted by a drum which is driven, so that it rotates about its own axis, by a variable-speed electric motor. The yarn arriving from the reel is wound in each case on this drum and is then unwound to feed the hosiery knitting machine.
This device is generally provided with a load cell arranged along the path followed by the yarn between the drum of the yarn accumulation device and the hosiery knitting machine for detecting the tension of the yarn. The load cell is connected to a comparator circuit which is in turn connected to the electric motor that drives the drum, so as to vary the rotation rate of the drum when the actual tension of the yarn that unwinds from the drum is different from a preset tension value, so as to bring the actual tension of the yarn to the preset value.
This device, installed in circular hosiery knitting machines, allows to produce tubular items in which the tension of the elastic yarn is practically constant over the entire length.
This device, however, has the limitation that it does not allow to produce items in which the tension of the elastic yarn is differentiated in the various regions of the item; i.e., it does not allow to vary the tension of the yarn during the manufacture of the item.
These devices also entail the problem that it is not possible to produce a high level of tension when feeding yarns that have a relatively large diameter.
Devices are also known which are constituted by so-called "positive feeders", i.e., feeders composed of two mutually opposite and contrarotating rollers which have parallel axes and between which the yarn to be fed passes. At least one of the two rollers is driven so as to revolve about its own axis by means of a variable-speed electric motor which is actuated with a speed correlated to the operating speed of the machine, so as to ensure the intended tension of the elastic yarn during its feeding.
With these devices it is possible to produce items in which the degree of tension of the elastic yarn can vary during the production of the item by varying according to requirements the actuation speed of the electric motor.
However, these devices entail the problem that they cannot precisely ensure a given tension of the elastic yarn, since when the unwinding conditions of the reel of yarn sent to the positive feeder vary, the degree of tensioning of the yarn sent to the hosiery knitting machine varies undesirably.